1. Field of the Invention
The present invention relates to a method of fabricating a miniaturized heterojunction bipolar transistor wherein at least one of an emitter layer and a collector layer is formed of a semiconductor material having a greater band gap than a material of a base layer.
2. Description of the Related Art
Recently, heterojunction bipolar transistors have been regarded as high-performance transistors, and, in particular, heterojunction bipolar transistors using a compound semiconductor have been widely studied. The study for the technique of incorporating a heterojunction in a silicon-based bipolar transistor has been progressing. Silicon-based heterojunction bipolar transistors are disclosed, for example, in (1) 1988, IEDM Digest of Technical Papers, p. 566, J. F. Gibbons et al. and (2) 1989, Symp. VLSI Tech. Digest of Technical Papers, p. 95, G. L. Patton et al. In these documents, it is shown that a base layer is formed of a silicon germanium alloy layer (SiGe layer), which is a strained epitaxial layer having a smaller band gap than silicon.
The conventional silicon-based heterojunction bipolar transistor has the following drawbacks. In the device of document (1), a junction area between an external base layer and a collector layer is large, and a collector-base junction capacitance is large. This makes it impossible for a transistor to operate at high speed. In the device of document (2), an element region is defined by an insulating film, and thereby a base-collector junction capacitance is made smaller than in the case of document (1). In the device of document (2), however, a large stepped portion is produced in the emitter region, and the possibility of breakage of electrode wiring must be taken into consideration. In addition, the external base layer and the emitter layer formed of polycrystalline silicon do not have a self-aligned structure. Consequently, it becomes difficult to control the distance between the external base layer and the emitter layer, and the increase in this distance raises the base resistance.